Gas turbine engines are typically surrounded by a fan casing which may be formed of a composite material comprising carbon fibres and a plastic material, e.g. bismaleimide (BMI) resin. To provide structural rigidity to the fan casing, an annular reinforcing rib is provided.
Such fan casings can be required to be fire resistant and it has been found that fan casings having a reinforcing rib formed of a carbon fibre/BMI resin composite tend to fail the industry requirements for fire resistance—it has been observed that significant flame is sustained by the composite reinforcing rib after removal of the heat source and this is unacceptable. It is thought that this is a result of the thickness of the reinforcing rib leading to a pool of resin that is too great to be burned-off during the fire resistance tests.
The annular reinforcing rib has a thickness that varies circumferentially. Attempts to replace the thicker portions of the composite reinforcing rib with metal (e.g. titanium) portions bolted to the composite reinforcing rib has been found to lead to an undesirable increase in fan casing weight, cost and part count.
The inventors have found that reducing the thickness of the thicker portions of the composite reinforcing rib and reinforcing them with laterally opposed metal (e.g. titanium) plates bolted to one another through the rib is unsuccessful—it is thought that the metal plates shield the composite reinforcing rib from the flame but act as a heat sink to sustain resin ignition as it melts and moves to the surface of the reinforcing rib through the bolt holes.
Mounting pads are also provided on fan casings for mounting/supporting accessories from the fan casing. These pads (also known as “pad up patches”) typically comprise thickened portions of the fan casing and, again, these thickened portions can result in a reservoir of resin that is too great to be burned off during the fire resistance test.
There is the need to provide a fan casing having a projection which meets the industry standards for fire resistance but which also has an acceptable weight and cost and is ideally integral to the fan case for a low part count.